Method for the electrolytic polishing of zirconium, hafnium and their alloys



United States Patent 3,234,111 METHOD FOR THE ELECTROLYTIC POLISHING OF ZIRCONIUM, HAFNIUM AND THEIR- ALLOYS Janis Alfreds Sietnieks, Hagersten, Sweden, assignor to Aktiebolaget Atomenergi, Stockholm, Sweden, :1 Swedish company No Drawing. Filed Sept. 25, 1961, Ser. No. 140,241 Claims priority, application Sweden, Sept. 27, 1960,

4 Claims. (Cl. 204-56) This invention relates to 'a method for an electrolytic surface treatment of objects consisting of zirconium, hafnium, and alloys containing one or both of said metals, in which the object to be treated is used as anode in an electrolyte containing ammonium fluoride. The invention particularly relates to the surface treatment of objects intended to be used as structural elements in an atomic reactor, where the'anodic treatment is to be followed by a so-called black-oxidation, i.e. an oxidation in steam at an increased temperature and pressure.

Black-oxidation is described in B. Lustman and F. Kerze: Metallurgy of Zirconium, McGraw-Hill Comp. Inc. (1955), pp. 608-640, and Hilding J. Mogard and Bengt T. Nelson; Fuel Elements in Sweden, 1961, Nuclear Engineering, vol. 6, November 1961, pp. 469-471.

It has been found that the known anodic surface treatment of objects consisting of zirconium, hafnium and their alloys in an aqueous solution of ammonium fluoride meets with certain difficulties. It appears that said difficulties are due to the fact that the metal surface is strongly attacked by the electrolyte. Therefore certain undesired changes of the metal surface take place after the object has been removed from the electrolyte, and before it has been dipped in a rinsing bath.

It has been found, according to the present invention that an addition of ammonium nitrate to the ammonium fluoride solution, which is substantially neutral, modifies the action of the electrolyte so that there does not take place any uncontrolled attack upon the metal surface within a reasonable time after the object has been lifted out of the electrolytic bath.

It appearsthat the favorable action of the nitrate is due to its stabilizing effect upon the oxidic layer formed on the metal surface and therefore that the nitrate exerts a passivating; action. Fluorides have a dissolving action on this oxidic layer and consequently corrode the metal.

The method of this invention has a particular interest when it is used as a pre-treatment to be followed by the known black-oxidation. This black-oxidation comprises an oxidation in steam at an increased pressure and temr perature, for instance a pressure of about 100 kilograms per square centimeter and a temperature of about 400 C. Under favorable conditions there is formed on the metal surface a black, tight oxidic film, which protects the metal from further attack. It is necessary, however, that the metal surface was duly treated before the blackoxidation, viz. thoroughly cleaned, polished and preferably passivated. If the surface is not thus treated, the surface film will consist of a white oxide which adheres poorly to the surface and does not give a good protection against corrosion. It has been found to be diflicult to produce a metal surface suitable for a subsequent black-oxidation when using the conventional anodic surface treatment in pure fluoride solutions. The method of this invention provides an excellent pre-treatment for the black-oxidation, which produces an oxidic film providing a good protection against corrosion.

According to this invention the electrolyte shall preferably contain 200-400 grams of ammonium fluoride per liter, and 200-400 grams of ammonium nitrate per liter. The total amount of fluoride and nitrate shall preferably be such as to give a substantially saturated solution,

3,234,111 Patented Feb. 8, 1966 'ice i.e. approximately 600 grams per liter. The objects to be treated are arranged as anodes in the bath. The current density shall be in the range of 0.5 to 3 and preferably 1 to 2 amperes per square decimeter. The best polishing result is obtained if the objects, preferably having acylindric shape, are slowly rotated in the bath for a time of 4 to 10 minutes. The temperature shall preferably not be too much above the normal room temperature, as the electrolyte may otherwise etch the metal surface. It appears that the optimal temperature is approximately 30 C.

The pH-value of the bath shall preferably be 6-8.4. It appears that the optimal pH-value is 7-8. As NH is formed during the process the pH-value has a tendency to rise. In order to keep the pH-value within the optimal range the NH may be neutralized, for instance, by the addition of nitric acid. I prefer, however, to control the pH-value by blowing a stream of a gas through the electrolyte. The fine gas bubbles dissolve gaseous NH and consequently remove NH from the bath. The gas may be air or nitrogen or any other gas (different from ammonia gas) which is substantially inert to the electrolyte.

After the anodic treatment the metal surface is passive to the electrolyte, and the object can be removed from the bath into a rinsing bath without any hurry and without voltage being applied. The interval between the electrolysis and the rinsing, the object being kept in the air, may last for several minutes without impairing the result. This is very useful when treating big objects. The rinsing is preferably done while rotating the objects, preferably at a comparatively high speed of rotation. After having been rinsed in water, the objects are preferably rinsed in an organic solvent such as alcohol, so as to remove the water. Objects having been thus pre-treated produce in the subsequent black-oxidation a shining, black, uniform and strongly adhering oxidic film having an excellent resistance to corrosion in an atomic reactor.

The method of this invention has a particular interest when treating such zirconium alloys that contain a small amount of tin, for instance the alloy called Zircaloy-2, which contains approximately 1.5% tin. The ammonium nitrate dissolves the tin, leaving a bright, metallic surface, whereas an electrolyte which merely contains ammonium fluoride does not dissolve the tin which forms a black coating on the surface of the alloy.

Example This example relates to the anodic treatment of fuel rods for an atomic reactor. The rods each consisted of a tube made from the alloy mentioned above, called Zircaloy-2, and the tubes contained the fissionable material. The rods each had a length of 773 millimeters and an outer diameter of 18.6 millimeters. Only the outer surfaces of the Zircaloy tubes were to be treated.

The anodic treatment was carried out in a cylindrical stainless steel vessel having a height of 1000 millimeters and a diameter of 300 millimeters, its volume thus being about 70 liters. The vessel was filled with an electrolyte consisting of distilled water containing ammonium fluoride in a quantity of 450 grams per liter electrolyte and ammonium nitrate in a quantity of grams per liter electrolyte. The temperature of the electrolyte was maintained at 2931 C. during the process.

Four fuel rods of the kind described were arranged in the electrolyte in said vessel. The rods were rotata-bly arranged in a frame, and were rotated around their own axes at a rate of 10 rpm. The rods were connected to the anode of a D.-C. source having a voltage of 6.4 volts, and the stainless steel vessel was connected to the cathode. Said voltage resulted in a current of 34 amperes, making a current density of 1.9 amperes per square decimeter of the surface of the fuel rods.

During the process air was introduced through a nozzle in the bottom of the vessel, thus bubbling through the electrolyte, keeping the pH-value at 7.9-8.2.

rods was lifted up from the vessel and lowered into a second vessel where the rods were rinsed with runningsisting of zirconium, hafnium and alloys containing av major amount thereof, as anode inan electrolytic bath consisting essentially of water, ammonium fluoride and ammonium nitrate,-said bath being maintained at a temperature of approximately 30 C. andat a pH of 6 to 8.4, to a current density of 0.53 amperes per square decimeter for from 4 to 10 minutes.

2.: A method as claimed in claim 1 in which the electrolyte contains about 200-400 grams of ammonium 4 i fluoride per literand about 200 -400 grams .of ammonium nitrate per liter, the total salt contentbeingabout; 600 grams per liter.

3. A method as claimed in claim 1 in which the objects being treated are slowly rotated inthe electrolytic bath.

4. A method as claimed in claim 1-i in wh-ich the pH -value of the electrolytegis maintained within the range from 7.9 to 8.2 by blowing a stream ofa gas different from ammonia gasand which is substantially inert to the; electrolyte through; the electrolyte.

References. Cited by-the Examiner UNI-TED? STATES PATENTS I 2,775,553f 12/,1956 wK'ahan; 204'- 56 WINSTON A; D OUGLAS, Primary Examiner. JOSEPH REBOLD'-,'J,OHN' H; MACK, Examiners. R. HARDER, :ALLEN Bl CURTIS, Assistant Examiners;

9/ 1959 McCord et'al. 204-.-;1,40.5 

1. A METHOD OF ELECTROLYTIC POLISHING OF ZIRCONIUM, HAFNIUM AND THEIR ALLOYS WHICH COMPRISES SUBJECTING AN OBJECT CONSISTING OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM AND ALLOYS CONTAINING A MAJOR AMOUNT THEREOF, AS ANODE IN AN ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF WATER, AMMONIUM FLUORIDE AND AMMONIUM NITRATE, SAID BATH BEING MAINTAINED AT A TEMPERATURE OF APPROXIMATELY 30*C. AND AT A PH OF 6 TO 8.4, TO A CURRENT DENSITY OF 0.5-3 AMPERES PER SQUARE DECIMETER FOR FROM 4 TO 10 MINUTES. 